Sputtering is a well established technology in the fabrication of silicon integrated circuits in which a metal target is sputtered to deposit target material onto the silicon wafer. In recent years, sputtering has also been applied for similar purposes in the fabrication of flat panel displays, such as flat computer displays and large flat televisions and the like. Various types of flat panel displays may be fabricated typically including thin film transistors (TFTs) formed on large thin insulating rectangular substrates, often called panels, such as glass or polymer and including liquid crystal displays (LCDs), plasma displays, field emitters, and organic light emitting diodes (OLEDs). Almost all panel fabrication equipment is distinguished by its large size. The original generation was based on panels having lateral dimensions of the order of 480mm: The newest generation contemplates panel sizes above 2m on a side. This large size has introduced several problems not experienced in wafer fabrication equipment limited to sizes of about 300mm in the most recent equipment.
A conventional flat panel sputter reactor 10 is schematically illustrated in the cross-sectional view of FIG. 1. Demaray et al. disclose more details of such a reactor in U.S. Pat. No. 5,565,071, incorporated herein by reference. A pedestal 12 within a main vacuum processing chamber 14 supports a panel 16 to be sputter coated in opposition to a target 18 bonded to a backing plate 20, which is sealed to but electrically isolated from the processing chamber 14. The target 18 may include one or more target tiles bonded to the backing plate 20. Argon sputtering gas is admitted to the main chamber 14 from an argon gas source 22 through a mass flow controller 24. A high-vacuum pump 26, for example, a cryo pump, is connected to the vacuum processing chamber 14 through a gate valve 28 and maintains a base pressure within the processing chamber 14 in the range of about 10−6 to 10−7 Torr but the argon pressure is typically kept in the milliTorr range for plasma sputtering. An unillustrated DC power supply electrically biases the target 18 to a sufficiently large negative voltage with respect to the pedestal 12 or the processing chamber 14 or a shield contained therein to excite the argon into a plasma. The positive argon ions are strongly attracted to the negatively biased target 18 to sputter material from it, which then strikes the panel 16 and coats material of the target onto the panel 16. In reactive sputtering, a reactive gas such as nitrogen is additionally admitted into the processing chamber 14 for react with sputtered metal to form a metal nitride. Still, the chamber pressure is maintained in the milliTorr range or below.
A magnetron 30 having opposed magnetic poles is positioned in back of the target 18 and backing plate 20 to create a horizontal magnetic field across the sputtering face of the target 18, which thereby intensifies the plasma and increases the sputtering rate. The form of the magnetron 30 is typically considerably more complex than that illustrated. The magnetron 30 is scanned in one or two dimensions about the back of the backing plate 20 to provide uniform deposition and target erosion. In wafer sputter reactors, the magnetron 30 is typically enclosed in a cooling water bath, which cools the target 18, which can become very hot under continued sputtering. However, such a configuration is unpractical with sputter reactors for large panels. The backing plate 20 and target 18 must be relatively thin to allow the magnetic field from the backside magnetron 30 to project through them. However, the very large size of the target means that the backing plate 20 would need to withstand an enormous force between the high vacuum within the vacuum processing chamber 14 and atmospheric pressure (760 Torr or 14 pounds per square inch) plus the weight and hydrostatic pressure of the cooling water. This problem is circumvented by forming liquid cooling channels within the relatively thin backing plate 20 and supplying coolant from outside the processing chamber 14 to cool the backing plate 20 and attached target 18. A magnetron chamber 32 surrounding the magnetron 30 is formed at the back of the backing plate 20 and is vacuum sealed to it. The magnetron chamber 32 is vacuum pumped, for example, to less than 1 Torr, more typically 200 to 500 milliTorr, thereby greatly reducing the force that the backing plate 20 must stand off.
Conventionally, a mechanical roughing pump 34 pumps the magnetron chamber 32 through a roughing valve 36 to the requisite sub-Torr pressure and maintains it there during panel processing. The cryo pump 26 is a high vacuum pump which is very effective at very low pressures but cannot be operated at pressures very much above 1 Torr. Therefore, when the main chamber 14 is being pumped down from atmospheric pressure at the beginning of operation, the same roughing pump 34 also pumps the processing chamber 14 through a second roughing valve 38 while the cryo gate valve 28 is closed. When the roughing pump 34 reduces the pressure in the main chamber 14 to a cross-over pressure of about 200 to 500 milliTorr, the second roughing valve 38 is closed and the cryo gate valve 28 is opened to allow the cryo pump 26 to further reduce the pressure within the main chamber 14 to the requisite pressure below 10−6 Torr. But, the first roughing valve 36 remains open so that the roughing pump 34 continues to pump the magnetron chamber 32 to a sub-Torr pressure.
In operation, panels 16 are inserted into the main chamber 14 through a slit valve 40 from a central transfer chamber held in the range of about 10−3 to 10−6 Torr. For an in-line system, two slit valves are positioned on opposed sides of the main chamber 14 to allow panels 16 to pass along a line of vacuum isolated chambers. The invention is not limited by the configuration of the larger system incorporating the sputter reactor of the invention.
The conventional pumping system, however, suffers several drawbacks. During pump down from 760 Torr, the pressure differential across the backing plate 20 must be minimized, for example, to less than 20 Torr. Pumping rates need to be matched between the two chambers 14, 32 and their operation needs to carefully timed to avoid excessive differential pressure across the target backing plate 20 during pump down. Also, when the main chamber 14 is vented to atmosphere for maintenance, the two chambers 14, 32 need to be brought up to atmospheric pressure at the same rate. Failure or miscontrol of one or the other valves 36, 38 can cause quick excursion of pressure in only one of the chambers 14, 32. Pump failure must be immediately addressed because atmosphere may back flow through either failed pump 34, 26, quickly increasing the pressure differential. A chamber leak needs to be quickly detected and pumping adjusted accordingly. In case of power or computer failure, the states of the valves 28, 36, 38 and pumps 26, 34 become uncertain. Any pressure differential of greater than about 20 Torr at a maximum will cause the backing plate 20 to excessively bend, perhaps causing the bonded target 18 or multiple target tiles to delaminate from the backing plate 20. Large pressure differentials may cause the backing plate 20 to permanently deform and even fracture. Large sputtering targets, particularly of refractory materials such as molybdenum, are exceedingly expensive and target failures due to pumping accidents need to be minimized.